A lens device that allows one portion of a plurality of solid-state lens combined with one another to move in a light axis direction has been known as a typical conventional varifocal lens device. In this system, however, since the moving distance in the light axis direction is limited, it is difficult to make the device thinner, and, for this reason, a varifocal lens that us as a liquid lens capable of achieving a thinner structure has been proposed.
With respect to the conventional varifocal lens that uses a liquid lens, a structure has been proposed in which a liquid droplet, formed by an interface between two liquids having different refractive indexes, is used is a lens, with one of the liquids being made to have an ion conductive property and the other liquid being made to lave an insulating property, and by applying a voltage between an electrode placed on the interface of these two liquids and an opposing electrode placed in the ion conductive liquid, the shape of the droplet is changed. For example, Patent Document 1 has disclosed a liquid lens having this structure. Such a change in the shape of the two-liquid interface upon application of a voltage is made mainly based upon a driving principle in which the surface tension balances of liquid/liquid/solid (electrode) interfaces made of the two-liquid interface and the electrode are changed by the voltage application, and this phenomenon is referred to as an electro-wetting phenomenon. In a typical embodiment of Patent Document 1, the applied voltage is set to 250V, and a comparatively high voltage application is required.
Moreover, Patent Document 2 has disclosed another varifocal lens device that utilizes the electro-wetting phenomenon. In an attempt to achieve superior adjusting performances of the lens shape upon application of a voltage, this Patent Document has disclosed a structure in which, in order to reduce hysteresis and stick slip of the contact angle between the two-liquid interface, an insulating layer is formed on an electrode surface, with a lubricating layer that contacts with the liquid interface being formed thereon. In one example of the embodiments of Patent Document 2, a thin film of highly fluorinated polymer is formed on a polyimide dielectric layer. The change in contact angle of the two-liquid interface is simultaneously accompanied by position shifts among the liquid/liquid/solid interface line; therefore, the controlling operations need to be executed on the surface with sufficiently controlled wettability, and since the controlling operations depend on very small surface tension balances, the resulting system has an issue of weakness to disturbance due to an external force.
Patent Document 1: Japanese Patent Publication of Japanese translation of PCT International Application No. 2001-519539
Patent Document 2: Unexamined Japanese Patent Publication No. 2003-177219